The excellent metallurgical wetting of 63% Sn and 37% Pb is thought to be promoted by the instantaneous formation of a thin layer of a very stable intermetallic compound at the molten solder interface. The interfacial intermetallic compound that aids solder wetting is always based on Sn, not Pb. The role of Pb in promoting solderability is much less understood, but seems related to its ability to strongly suppress the liquid surface tension of solder alloy, lowering the contact angel of the molten solder which leads to better spreading and more interacting surface area for the solder joint to form. The eutectic solidification reaction of 63% Sn and 37% Pb also generates a highly refined mixture of Sn and Pb phase that can produce unusual strength from rather weak constituents.
In response to the possibility of a governmental ban or excessive taxation of Pb used, manufactures and users of Pb-bearing solder for electronic and circuit joining application recently have attempted to develop Pb-free solders as direct replacements for the conventional Sn—Pb solders in conventional hand/wave/reflow soldering processes employing general heating of an electronic wiring board to temperatures of 240 to 250 degree Celsius. Moreover, there is a need for such a solder that would be competitive in cost and as readily available as conventional Sn—Pb eutectic solders for high volume use.
A ternary, off-eutectic Sn—Ag—Cu solder alloy has been developed by the plumbing industry as a Pb-free solder with 96 weight % Sn, 3.5 weight % Cu, and 0.5 weight % Ag. This plumbing solder has a solidus temperature of 227 degree Celsius and a liquidus temperature of 260 degree Celsius, which would be considered not suitable for electronics manufacturers (U.S. Pat. No. 5,527,625, Anderson et al. Jun. 18, 1996).
Recent developments of Pb-free solder alloys mainly involve the use of Ag, Bi, Cu, In, Sn and Zn. Special attentions have been drawn on the solder composition comprising Sn, Ag, and Cu. Anderson et. al. of Iowa State University Research have reported the eutectic Pb-free solder comprising 93.6Sn/4.7Ag/1.7Cu and Sn/Ag/Cu/Ni/Fe, respectively (U.S. Pat. No. 5,527,625, Anderson et al. Jun. 18, 1996). As the solder based on the Sn—Ag—Cu system, Anderson et al. have provided a Pb-free solder comprising 93.6Sn/4.7Ag/1.7Cu, which is more excellent than other Pb-free solder in welding performance of metal, conductivity, mechanical property, pliability and so on, but has a poorer wetting property, and a coarse alloy structure, as a result, it's distribution is not easy to be even. On the other hand, since the content of Ag is high up to 4.7 weight % in the Pb-free solder based on the Sn—Ag—Cu system, the silver will be reacted with components of the soldering flux, which contain halogen, to produce silver bromide, silver chloride, silver fluoride, or silver carbonate during its application. After such silver compounds are exposed to light, a phenomenon of changing to black occurs, as a result, the final products concerned the Pb-free solder are trend to change to black. In addition, Hu Jianxiong et al. have reported that chemical molecules of P, which has non-metal property, will penetrate into the structure of metal molecule of Pb-free solder under P are molten, thereby to improve better micro-structural stability and hence reduce the formation of dross (State Intellectual Property Office of People's Republic of China: Zl Patent No. 03110895.4, Hu Jianxiong et al., Jan. 15, 2003).
It is therefore an object of the present invention to provide a new and improved composition of an environmental friendly Pb-free solder alloy that has all of the advantages of prior art and yet keep our environment green.
It is another object of the present invention to provide a new and improved composition of an environmental friendly Pb-free solder alloy that may be efficiently manufactured and marketed.
An even further object of the present invention is to provide a new and improved composition of an environmental friendly Pb-free solder alloy that is susceptible of a low cost of manufacture with regard to both materials and labor, and which accordingly is then susceptible of low prices of sale to the consuming public, thereby making such composition available to the buying public.
Still yet a further object of the present invention is to provide a new and improved composition of an environmental friendly Pb-free solder alloy which can be used to make Pb-free solder bar, wire, BGA ball and anode in electronics industries.
These together with still other objects of the invention, along with the various features of novelty that characterize the invention, are pointed out with particularity in the claims forming a part of this disclosure. For a better understanding of the invention, its operating advantages and the specific objects attained by its uses, reference should be made to the following detailed description of the preferred examples of the invention.